The mechanisms involved in the establishment of the hematopoietic receptor are poorly understood, largely due to the inaccessibility of the appropriate cell populations with the early embryo at the onset of hematopoiesis. Embryonic stem (ES) cells can spontaneously differentiate and generate hematopoietic cells in cultured and as such, provide a unique model system with which to study early hematopoietic development. The overall goal of this proposal is to utilize their system to analyze the earlier events involved in the establishment of the hematopoietic system. Towards this goal, the first part of the proposal focuses on the characterization of the growth requirements and developmental potential of the earliest hematopoietic cells to develop from the ES cells in cultures. The second part of the proposal is aimed at analyzing the B-lymphocyte and hematopoietic stem cell potential of populations that arise late in the differentiation of cultures. The final part of the proposal focuses on defining molecular events involved in the onset of hematopoietic analyzing the genes expression pattern of the early hematopoietic cells as well as the developmental potential of ES cells lacking specific genes. The outcome of these experiments will provide a clearer understanding of the characteristics of the earliest embryonic hematopoietic cells and of how these cells relate to later populations. In addition, they will allow us to define the role of specific genes in the establishment of various hematopoietic populations in cultures.